1. Field of the Invention
The present invention relates to an image heating apparatus of a film heating system applied to image forming apparatuses such as a copying machine and a printer, particularly to a heater applied to an image heating apparatus.
2. Related Background Art
In conventional image forming apparatuses such as a printer, a copying machine and a facsimile apparatus, as a fixing apparatus (fixing device) for heating/fixing an unfixed image (toner image) formed and borne on a recording material (transfer material, photosensitive paper, electrostatic recording paper, printing sheet, and the like) by appropriate image forming means such as an electrophotographic system and an electrostatic recording system in a transfer (indirect) system or a direct system, an apparatus of a heat roller system is widely used.
The apparatus of the heat roller system has a fixing roller (thermal roller, heat roller) as a fixing member and a pressure roller as a pressurizing member which are pressed to contact each other and rotate. When a recording material with an unfixed image formed and borne thereon is introduced, nipped, conveyed, and passed via a fixing nip portion (heating nip portion) as a pressed portion of both rollers, the unfixed image can be heated/fixed as a permanent fixed image on a recording material surface by the heat of the fixing roller and the pressurizing force of the fixing nip portion.
In recent years, from the standpoint of promotion of energy saving, an apparatus of a film heating system has been placed for practical use as an on-demand image heating apparatus high in thermal conduction efficiency and fast in starting the apparatus.
As proposed in Japanese Patent Application Laid-Open Nos. 63-313182, 2-157878, 4-44075 to 4-44083, and 4-204980 to 4-204984, this has a fixed/supported heating member, a heat resistant film which slides on the heating member, and a pressurizing member contacting the heating member via this film to form a fixing nip portion. The heating member is heated/adjusted to a predetermined temperature, and a recording material with an unfixed image formed/borne thereon is introduced between the film and the pressurizing member at the fixing nip portion, and nipped/conveyed with the film through the fixing nip portion, so that the unfixed image is heated/fixed as a permanent fixed image on a recording material surface by the heat from the heating member via the film and the pressurizing force of the fixing nip portion.
In the image heating apparatus of the film heating system, a linear heating member with a low thermal capacity such as a so-called ceramic heater as the heating member, and a thin heat resistant film with a low thermal capacity as the heat transfer member can be used. The temperature of the heating member is raised in a short time, and the rising of the temperature of the heating member or the fixing nip portion to a predetermined temperature can quickly be performed. No power is supplied to the apparatus (heating member) during standby, and the power consumption can be minimized. Therefore, as compared with the other image heating apparatus of the heat roller system or the like, power can be saved and wait time can be shortened (quick start property), so that the on-demand image heating apparatus can be constituted.
FIG. 10 is a schematic view showing a main part of one example of the image heating apparatus (heating/fixing apparatus) of the film heating system.
Specifically, the image heating apparatus has a heating member 11 (hereinafter referred to as the heater) fixed/supported on a stay holder (heater supporter) 12, and an elastic pressure roller 20 held and pressed onto the heater 11 via a heat-resistant thin film 13 (hereinafter referred to as the fixing film) to form a fixing nip portion N with a predetermined nip width.
When electricity is supplied, the heater 11 is heated to a predetermined temperature, and the temperature is adjusted.
The fixing film 13 is a cylindrical member, an endless belt-like member, or a rolled web-like member having ends. The film is attached and slid onto the surface of the heater 11 in the fixing nip portion N, and conveyed/moved in a direction of arrow a.
When the heater 11 is heated to the predetermined temperature, the temperature is controlled, and the fixing film 13 is conveyed/moved in the direction of arrow a, a recording material P with an unfixed toner image t formed/borne thereon is introduced as a material to be heated between the fixing film 13 and the pressurizing roller 20 of the fixing nip portion N. Then, the recording material P is attached to the surface of the fixing film 13, and held/conveyed with the fixing film 13 through the fixing nip portion N.
In the fixing nip portion N, the recording material P with the toner images t is heated by the heater 11 via the fixing film 13 so that the toner images t on the recording material P are heated/fixed.
The recording material portion passed through the fixing nip portion N is peeled off from the surface of the fixing film 13 and conveyed.
A ceramic heater is usually used in the heater 11. FIG. 11A is a partially cut plan model view showing the front surface side (heating surface side) of the ceramic heater 11, and FIG. 11B is a plan model view of the rear surface side (surface side opposite to the heating surface).
Specifically, for example, the front surface side (surface on the side facing the fixing film 13) of a ceramic substrate 11a of alumina having electric insulation properties, good thermal conductivity, and a low thermal capacity is provided with a energizing heating resistance layer (heating member) 11b of Ag/Pd (silver palladium), Ta2N, and the like formed along the longitudinal direction of the substrate by screen printing or the like. Furthermore, the surface with the energizing heating resistance layer formed thereon is covered with a thin glass protective layer 11c. For the heater 11, by supplying power via a power supplying electrode portion 11d, the energizing heating resistance layer 11b is heated so that the temperature of the entire heater is rapidly raised.
The temperature rise of the heater 11 is detected by temperature detecting means 14 disposed on the heater rear surface, and fed back to a energizing controller (not shown) via electric path patterns 11e, through holes 11f, and electrode portions 11g for output to a temperature controller.
The energizing controller controls the energizing of the energizing heating resistance layers 11b so that the heater temperature detected by the temperature detecting means 14 is maintained at a substantially constant predetermined temperature (fixing temperature). Specifically, the heater 11 is heated and controlled or adjusted to the predetermined fixing temperature.
The fixing film 13 is formed to be remarkably thin as 20 to 70 xcexcm in order to efficiently give the heat of the heater 11 to the recording material P as the material to be heated in the fixing nip portion N. This fixing film 13 is constituted of three layers, that is, a film base layer, a primer layer, and a mold release layer, the film base layer is on the side of the heater 11, and the mold release layer is on the side of the pressurizing roller 20. The film base layer is formed of polyimide, polyamide-imide, PEEK, or the like which is higher in insulation property than the glass protective layer 11c of the heater 11, and has a heat resistance and a high elasticity. Moreover, the mechanical strengths such as tear strength of the entire fixing film 13 are kept by the film base layer. The primer layer is formed of a thin layer which has a thickness of about 2 to 6 xcexcm. The mold release layer is a toner offset preventive layer to the fixing film 13, and is formed by coating fluoroplastics such as PFA, PTFE and FEP in a thickness of about 10 xcexcm.
Moreover, the stay holder 12 is formed, for example, of a heat-resistant plastic member to hold the heater 11, and also serve as a conveyance guide of the fixing film 13.
In the heating apparatus of the film heating system using such thin fixing film 13, the pressurizing roller 20 having an elastic layer is flatted along the lower flat surface of the heater 11 pressurized via the film 13 by a high rigidity of the ceramic heater 11 in the pressurizing portion to form the fixing nip portion N with the predetermined width, and only the fixing nip portion N is heated to realize a quick start heating/fixing.
Character S denotes a recording material conveying standard (sheet passing standard), and in the apparatus of the example, the standard is disposed in the middle of the recording material conveying area of an image forming apparatus main body in the longitudinal direction. The apparatus has a xe2x80x9ccentral standardxe2x80x9d.
The width of the energizing heating resistance layer 11b of the heater 11 in the longitudinal direction, that is, an effective heat generating area W is formed to be slightly narrower as compared with a width D (pressurizing roller abutting area) of the elastic layer of the pressurizing roller 20 which abuts on the heater 11 via the fixing film 13. This prevents a problem that the temperature locally rises and breakage is caused by thermal stress when the energizing heating resistance layer 11b is protruded from the pressurizing roller 20.
Moreover, the effective heat generating area W of the energizing heating resistance layer 11b is formed in a sufficiently broader width than that of an area for conveying sheets with normal sizes such as A4 and LTR, that is, a passing portion A (normal sized sheet passing portion, large sized sheet passing portion). This can eliminate the influence of end portion temperature sag (by heat leakage to electric contacts, connectors 31, 32, and the like on the end portions of the heater 11), so that effective fixing properties can be obtained over the entire surface of the recording material P.
Furthermore, in some cases, the width of the energizing heating resistance layer 11b on the end portion of the sheet passing area is shortened, and the heating value of the end portion is increased to compensate for the fixing properties of the end portions.
Therefore, the heat generated by energizing the energizing heating resistance layer 11b of the heater 11 is given to the recording material P conveyed between the fixing film 13 and the pressurizing roller 20, and acts to melt and fix the toner images t on the recording material P.
The temperature detecting element 14 such as a thermistor, and a thermo-protector 15 such as a temperature fuse and a thermo-switch for shutting down the energizing of the energizing heating resistance layer 11b of the heater 11 during runaway abut on the rear surface of the heater 11. The temperature detecting element 14 and the thermo-protector 15 are disposed in an area for conveying small sized sheets such as envelopes, that is, a small sized sheet passing portion B (minimum width recording material conveying area). The thermo-protector 15 is interposed in series with the power supply path to the energizing heating resistance layer 11b. 
Here, the temperature detecting element 14 is disposed in the small sized sheet passing portion B, so that even when the recording material P having the minimum width that can be conveyed in the image forming apparatus main body is conveyed, the toner image t on the recording material P is heated/fixed at an appropriate fixing temperature without causing any fixing failure, high temperature offset, or other problems.
On the other hand, the thermo-protector 15 is disposed in the small sized sheet passing portion B, so that when the recording material P with the minimum width is conveyed, in a non-conveying area, that is, a small sized sheet non-passing portion C which has a smaller heat resistance than the small sized sheet passing portion B as the conveying area, a problem that the thermo-protector 15 is incorrectly operated by overheating in the small sized sheet non-passing portion C to shut out the energizing even during normal conveyance, or other problems are prevented from occurring.
Additionally, since the thermo-protector 15 abuts on the rear surface of the heater 11, in some cases the heat amount generated in the energizing heating resistance layer 11b is taken by the thermo-protector 15, a sufficient heat amount cannot be applied to the recording material P, and fixing failure occurs in the abutting position of the thermo-protector 15. To prevent this, by slightly narrowing the energizing heating resistance layer 11b in the position corresponding to the abutting position of the thermo-protector 15 like 11bxe2x80x2 and by setting the resistance value of the energizing heating resistance layer 11bxe2x80x2 to be larger than the values of the other energizing heating resistance layer portions, the heat generating amount is secured. Thereby, the heat supply amount to the recording material P is set to be constant over the longitudinal direction of the heater 11, and excellent heating/fixing is realized without any fixing nonuniformity.
Since the temperature detecting element 14 also abuts on the rear surface of the heater 11 in the same manner as the thermo-protector 15, it is also feared that the heat generated by the energizing heating resistance layer 11b is taken by the temperature detecting element 14. However, by using the temperature detecting element 14 with a small heat capacity such as a chip thermistor, the heat amount taken from the heater 11 can be minimized. Therefore, even if the above-described countermeasure is not taken like in the thermo-protector 15, uniform fixing can be realized in the longitudinal direction of the heater without deteriorating the fixing uniformity of the recording material.
In the image heating apparatus of the film heating system as described above in the conventional example, when sheets (recording materials) different in size (sheet width) are passed, the heat amount taken from the heater differs in the sheet passing portion and the sheet non-passing portion. The temperature of the sheet non-passing portion in which heat is not taken by the sheet gradually rises as the sheets are passed (sheet non-passing portion temperature rise phenomenon), and finally exceeds the heat resistant temperatures of the heater, pressurizing roller, and heater holder. The problem is solved by enlarging the sheet passing interval.
However, in recent years, with the increase of adjustment temperature and input power for a higher speed printer, the temperature rise of the sheet non-passing portion has become more remarkable, which cannot be solved by the method of enlarging the sheet passing interval any more.
To solve this problem, zone heating is effective in which the heater is provided with a plurality of heating members (energizing heating resistance layers) different in heat generating area, and the heating/fixing is performed by changing the heating member to be heated in accordance with the sheet size.
FIGS. 12A, 12B and 12C are diagrams showing one example of the zone heating type heater 11 as the background art of the present invention. FIG. 12A is an enlarged transverse sectional model view of the heater 11, FIG. 12B is a plan model view of the rear surface side, and FIG. 12C is a pattern model view of a normal sized sheet heating member and a small sized sheet heating member.
The heater 11 in this example is a rear surface (back surface) heating type ceramic heater. Specifically, in the constitution, the substrate rear surface side (non-heating surface side) facing away from the front surface side (heating surface side, surface of the side facing the fixing film) of the highly heat conductive ceramic substrate 11a such as Al2O3 and AlN is provided with the heating member (energizing heating resistance layer such as Ag/Pb and Ta2N).
In the heater 11 of this example, a normal sized sheet heating member H1, and a small sized sheet heating member H2 parallel with the member H1 are formed along the longitudinal direction on the rear surface side of the ceramic substrate 11a. Power supplying electrode portions 11d1, 11d1 are energized and formed on both end portions of the normal sized sheet heating member H1. Power supplying electrode portions 11d2, 11d2 are energized and formed on both end portions of the small sized sheet heating member H2. The thin glass protective layer 11c is formed to cover the surface on which the normal sized sheet and small sized sheet heating members are formed. The temperature detecting means (thermistor) 14 and the thermo-protector 15 are disposed to contact the surface of the glass protective layer 11c on the rear surface side of the heater.
Character S denotes a recording material conveying standard (sheet passing standard), and in the apparatus of the example, the standard is disposed in the middle of the recording material conveying area of the image forming apparatus main body in the longitudinal direction. The apparatus has a xe2x80x9ccentral standardxe2x80x9d. Character X denotes a sheet passing direction.
The normal sized sheet heating member H1 is disposed for the recording materials of A4, LTR, LGL, and the like, its length L1 is set to 222 mm (equal to effective heat generating area W), and its width W1 is set to 3 mm.
The small sized recording material heating member H2 is adapted to the small sized sheet passing portion B for envelopes such as com 10, DL and monarch, its length L2 is set to 116 mm, and width W2 is set to 1.57 mm.
The temperature detecting element 14 and the thermo-protector 15 are disposed in the small sized sheet passing portion B.
When power is supplied between the power supplying electrode portions 11d1 and 11d1 during the passing of normal sized recording materials, the normal sized sheet heating member H1 is heated and the temperature of the entire heater is rapidly raised. This temperature rise of the heater 11 is detected by the temperature detecting element 14 and fed back to the energizing controller (not shown). The energizing controller controls the energizing of the normal sized sheet heating member H1 so that the heater temperature detected by the temperature detecting element 14 is maintained at a substantially constant predetermined temperature (fixing temperature).
When small sized recording materials are passed, power is supplied between the power supplying electrode portions 11d2 and 11d2, and the small sized sheet heating member H2 is heated. Subsequently, the temperature of the heater corresponding to the small sized sheet passing portion B is detected by the temperature detecting element 14 and fed back to the energizing controller. The energizing controller controls the energizing of the small sized sheet heating member H2 so that the heater temperature detected by the temperature detecting element 14 is maintained at the substantially constant predetermined temperature (fixing temperature).
However, when the zone heating is performed by independently energizing the heating members H1 and H2 different in heat generating area and by passing the sheets, temperature distributions h1 and h2 are formed in the sheet passing direction of the heater substrate as shown in FIG. 13. Specifically, when the heating member H1 positioned on the upstream side of the sheet passing direction is energized in the fixing nip portion N, the temperature distribution h1 is obtained in the sheet passing direction of the heater substrate, and the temperature in the fixing nip portion can be kept substantially uniformly. However, when the heating member H2 positioned on the downstream side is energized, the temperature distribution h2 is obtained in the sheet passing direction of the heater substrate, and a large temperature gradient is generated in the upstream/downstream direction in the fixing nip portion. This is because there is a large heat flux to the sheet from the heater on the upstream side on which sheet temperature is low, and there is a small heat flux on the downstream side on which the sheet temperature is high.
Therefore, when the temperatures of a plurality of heating members H1 and H2 are adjusted/controlled by one temperature detecting element (thermistor) 14, and when the heating members H1 and H2 are independently energized as described above, a moderate temperature gradient (usually the vicinity of temperature peak) differs with each case. Even when the temperature detecting element is placed substantially between the temperature peaks, a problem occurs that the detected temperature of the member H2 largely fluctuates within the attaching tolerance of the temperature detecting element.
Moreover, during the energizing of the heating member H2 positioned on the downstream side, since the entire heater substrate cannot be kept at a high temperature, there is a problem that the excellent fixing properties cannot be obtained.
An object of the present invention is to provide an image heating apparatus and an image heater in which heater temperature can correctly be detected even when there is an attaching position error of a temperature detecting element.
Another object of the present invention is to provide an image heating apparatus and an image heater in which a small sized recording material can sufficiently be heated.
Further object of the present invention is to provide an image heating apparatus comprising a heater having a long base material, a temperature detecting element for detecting temperature of the heater, and a film having one surface which slides on the heater and the other surface which moves while contacts a recording material bearing an image. The heater is controlled by an output from the temperature detecting element to obtain a predetermined temperature, the image on the recording material is heated by heat from the heater via the film, the heater has a first heating member disposed along a longitudinal direction of the base material and heated by energizing and a second heating member shorter than the first heating member, and the first heating member is disposed on the upstream side of the second heating member with respect to a moving direction of the recording material. When a first size recording material is heated, the first heating member is energized and the second heating member fails to be energized. When a second size recording material smaller than the first size recording material is heated, the first heating member and the second heating member are energized.
Another object of the present invention is to provide a heater for heating image comprising a long base material, a temperature detecting element for detecting temperature, a first heating member disposed along a longitudinal direction of the base material and heated by energizing, and a second heating member shorter than the first heating member. The first heating member and the second heating member are arranged in a direction orthogonal to the longitudinal direction of the base material, the first heating member is disposed for a first size recording material and a second size recording material smaller than the first size recording material, and the second heating member is disposed for the second size recording material.
Still another object of the present invention is to provide a heater for heating image comprising a long base material, a first heating member disposed along a longitudinal direction of the base material and heated by energizing, a second heating member shorter than the first heating member, and a third heating member having substantially the same length as the length of the first heating member. The first heating member, the second heating member and the third heating member are arranged in a direction orthogonal to the longitudinal direction of the base material, and the second heating member is disposed between the first heating member and the third heating member.
Further objects of the present invention would be apparent from the following description.